[llvm-dev] What's the most efficient way to load a vector of pointers?

[Craig Topper via llvm-dev]

For masked.gather, the mask should be all 1s to do a load for each element.
You should set the passthru to undef. On targets that don't have native
support for gather in hardware, the gather will be expanded into a loop
that does each load and inserts it into a vector. This is expansion is done
by the ScalarizedMaskedMemIntrinsicPass that runs a little before
SelectionDAG.

~Craig


On Tue, Apr 7, 2020 at 6:46 PM Levo DeLellis via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello everyone. I'll tell you my goal then my question. In pseudo code I'd
> like to do the following
>
> values = lookup_table[offsets]
>
> offsets is `<8 x i8>` which I get by doing `and <8 x i8> %a, %b`
> lookup_table is simply a 16 byte constant array of i8 aligned to 16.
> values would be `<8 x i8>` which I'll use with another vector operation
>
> I was thinking I might need an array of pointers but I looked at
> getelementptr for a solution. It appears it can use it
> https://llvm.org/docs/LangRef.html#vector-of-pointers
> Specifically the following line
>
> %A = getelementptr i8, i8* %ptr, <4 x i64> %offsets
>
> However it doesn't show how I should load %A but mentions gather (I don't
> need a mask or pass through values). I looked at 'load' and it doesn't
> appear to support a vector. Is there anything else I can use that may be
> more efficient? If llvm.masked.gather is the way to go do I want the mask
> all 1's or 0's? I heard of masked-off bit being different things to
> different people but my first impression is I want all 0's
>
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